1. Field of the Invention
The invention relates to an economical soft magnetic alloy with good temperature stability of the magnetic permeability and good resistance to oxidation in a damp environment. This alloy can be used, preferably, to manufacture the stator of an electric stepping micromotor for use in horology (the science of measuring time).
2. Description of the Background
Electric micromotors for use in horology comprise a stator, generally made of soft magnetic alloy containing about 80% nickel, a few percent of molybdenum or copper, the remainder being iron. An alloy of this kind has a maximum magnetic permeability of 200,000 to 300,000 across the entire range of operating temperatures (xe2x88x9220xc2x0 C., +60xc2x0 C.) and micromotors manufactured in this way therefore consume very little energy. However, alloys containing 80% nickel are expensive and readily oxidize in damp environments, and this presents a number of drawbacks: they are awkward to use in certain hot and humid regions; they are ill-suited to the manufacture of timepieces in which the mechanism is visible; and they are too expensive to use in the manufacture of economical timepieces.
In order to remedy these drawbacks, it has been proposed that alloys containing 80% nickel be replaced with alloys of the iron-nickel-chromium type containing less than 50% nickel and a few percent chromium for the manufacture of motors for timepieces. However, the alloys proposed generally have a magnetic permeability which is both too low and too sensitive to temperature. This excessive sensitivity of the magnetic permeability to temperature is a drawback. This is because the motor of a timepiece needs to operate satisfactorily between xe2x88x9220xc2x0 C. and +60xc2x0 C., which means that the magnetic permeability must not vary too much across this temperature range.
Bearing in mind all the constraints imposed on a stepping micromotor for use in horology, it is desirable for the manufacture of the stator of an economical motor of this type, to have a soft magnetic alloy with a magnetic flux density at saturation Bs greater than or equal to 5000 gauss (0.5 tesla), a maximum relative DC permeability xcexcDCmax greater than 70,000, sufficient resistivity xcfx81 that xcexcDCmaxxc3x97xcfx81 greater than 0.05 xcexa9.m, sufficient stability of the magnetic permeability xcexcDCmax between xe2x88x9220xc2x0 C. and +60xc2x0 C., improved resistance to oxidation and a relatively low nickel content. For the magnetic permeability to have sufficient stability, it is desirable that its variation, in terms of relative values, with respect to its value at 20xc2x0 C. remain lower than 30% across the temperature range considered.
Objects of the present invention include the provision an alloy which meets the above requirements, and the manufacture and use of such an alloy.
To this end, one embodiment of the invention is a soft magnetic alloy, the chemical composition of which comprises, consists of or consists essentially of in % by weight based on total weight:
34%xe2x89xa6Nixe2x89xa640%
7% Crxe2x89xa610%
0.5%xe2x89xa6Coxe2x89xa63%
0.1%xe2x89xa6Mnxe2x89xa61%
as well as iron and the usual impurities which result from the production process.
Preferably the impurities represented by O, S and N if present are such that:
Oxe2x89xa60.007%
Sxe2x89xa60.002%
Nxe2x89xa60.004%
and:
N+S+Oxe2x89xa60.01%.
It is also preferable that the impurities Si, Al, Ca and Mg if present be such that:
Sixe2x89xa60.3%
Alxe2x89xa60.05%
Caxe2x89xa60.03%
Mgxe2x89xa60.03%
and that:
Si+Al+Ca+Mg+Mnxe2x89xa61%
This alloy can be used in all applications known for soft magnetic alloys and preferably for the manufacture of a magnetic yoke and, in particular, the manufacture of the stator of an electric stepping micromotor for use in horology.
The invention will now be described in greater detail and illustrated by some examples.
The chemical composition of the invention soft magnetic alloy preferably comprises, consists of or consists essentially of, in % by weight based on total weight:
more than 34% nickel, in order to obtain sufficient magnetic flux density at saturation and magnetic permeability. However, to obtain an economical alloy, and in particular, bearing in mind the addition of chromium, the nickel content preferably is below 40%,
from 7% to 10% chromium, to improve the resistance to oxidation and increase the low-temperature magnetic permeability; when the nickel content is between 34% and 40%, a chromium content such as this appreciably improves the magnetic permeability between xe2x88x9240xc2x0 C. and 0xc2x0 C.,
from 0.5% to 3% cobalt, to obtain sufficient temperature stability of the magnetic permeability. Specifically, the inventors have found, unexpectedly, that for nickel contents of between 34% and 40% and chromium contents of between 7% and 10%, a modest addition of cobalt appreciably improved the temperature stability of the magnetic permeability between xe2x88x9220xc2x0 C. and 60xc2x0 C.,
from 0.1% to 1% manganese, and preferably more than 0.2%, to deoxidize the alloy and fix any sulfur.
the remainder of the composition comprises, consists of, or consists essentially of iron and of the usual impurities that result from the production process.
The impurities may be, in particular, oxygen, sulfur, nitrogen, silicon, aluminum, calcium and magnesium.
All of these impurities have a prejudicial effect on the magnetic properties and so, in order to obtain desirable magnetic properties, it is preferable that:
the oxygen content remain below or equal to 0.007%, the nitrogen content remain below or equal to 0.004%, the sulfur content remain below or equal to 0.002%, and the sum O+N+S of the oxygen, nitrogen and sulfur contents, remain below or equal to 0.01 %;
the residual contents of deoxidizing elements such as Si, Al, Ca, Mg remain below or equal to 0.3% in the case of silicon, 0.05% in the case of aluminum, and 0.03% in the case of calcium or magnesium; calcium and magnesium have the advantage of allowing the formation of small oxides which make the alloy easier to cut.
Furthermore, it is preferable for the sum Mn+Si+Al+Ca+Mg of the manganese, silicon, aluminum, calcium and magnesium contents to remain below or equal to 1%.
The contents of other impurities such as phosphorus and boron should also remain as low as possible.
The invention alloy thus defined, which is of Fexe2x80x94Nixe2x80x94Crxe2x80x94Co type, can be hot rolled then cold rolled and optionally annealed under hydrogen at a temperature of 900xc2x0 C. or higher for more than an hour, preferably between 1100xc2x0 C. and 1200xc2x0 C. for 1 to 4 hours. The high-temperature annealing under hydrogen has the advantage of at least partially eliminating certain sulfide or nitride precipitates which have a prejudicial effect on the magnetic properties.
The invention alloy preferably has a magnetic flux density at saturation Bs higher than 5000 gauss at 70xc2x0 C., a maximum relative DC magnetic permeability xcexcDCmax greater than 70,000 at 20xc2x0 C., an electrical resistivity xcfx81 greater than 70 xcexcxcexa9.cm at 20xc2x0 C., and temperature stability of the maximum relative magnetic permeability defined for a temperature T by:
|xcex94xcexcDCmax(T)/xcexcDCmax(20xc2x0 C.)|xe2x89xa630%
In this formula, xcex94xcexcDCmax(T) represents the variation in xcexcDCmax between 20xc2x0 C. and T, and xcexcDCmax(20xc2x0 C.) represents the DC permeability at 20xc2x0 C.
Furthermore, given its chromium content, the invention alloy has good resistance to oxidation in damp environments.